1. Field of the Invention
The present invention relates to a semiconductor device, for example, a power semiconductor device which realizes a low on-resistance.
2. Related Background Art
In recent years, an insulated gate bipolar transistor (IGBT) has broadly been used as a power semiconductor device which has a breakdown voltage of 600 V or more. Since this power semiconductor device is generally used as a switch, a low on-resistance and a high switching rate are requested.
An IGBT according to a conventional art will be described with reference to FIGS. 27 and 28. It is to be noted that in drawings described later, the same components are denoted with the same reference numerals, and the detailed description thereof is appropriately omitted.
FIG. 27 is a sectional view schematically showing one example of the IGBT comprising a trench gate structure which has recently been used broadly. An IGBT 110 shown in the figure comprises an n−-type base layer 142, a p+-type emitter layer 44, a collector electrode 56, p-type well layers 160, n+-type source layers 148, an emitter electrode 158, gate oxide films 52, and a gate electrode 54. The p+-type emitter layer 44 is formed on the undersurface of the n−-type base layer 142 on the lower side in the sheet of the figure, and the collector electrode 56 is disposed in contact with the p+-type emitter layer 44. The p-type well layer 160 is formed in the upper surface of the n−-type base layer 142 in the sheet of the figure. A trench TRa is selectively formed halfway in the depth of the n−-type base layer 142 from the surface of the p-type well layer 160 through the p-type well layer 160, and the gate electrode 54 is disposed via the gate oxide film 52 in the trench. In the n−-type base layer 142, a width 2a of an active region held between the gate oxide films 52 is usually, for example, 2a≅4 μm. The n+-type source layer 148 is selectively formed in contact with a trench TR52 in the surface portion of the p-type well layer 160. Furthermore, the emitter electrode 158 is disposed so as to extend to the surface of the n+-type source layer 148 from the upper surface of the p-type well layer 160, thereby the emitter electrode 158 contacts the p-type well layer 160 and n+-type source layer 148.
An operation of the IGBT 110 shown in FIG. 27 is as follows.
When a bias voltage which is positive with respect to the emitter electrode 158 is applied to the gate electrode 54, an inversion layer is generated in the p-type well layer 160 in a region in the vicinity of the outer surface of the gate oxide film 52, and electrons are injected into the n−-type base layer 142. Accordingly, a positive hole is injected into the n−-type base layer 142 from the p+-type emitter layer 44, which turns the IGBT 110 on. The positive hole injected at this time runs in the n−-type base layer 142 to flow into the p-type well layer 160.
However, the related-art IGBT 110 shown in FIG. 27 has the following two problems.
A first problem is that, due to the positive hole flowing into the p-type well layer 160, accumulation of the positive hole decreases in the vicinity of an interface between the n−-type base layer 142 and the p-type well layer 160, and carriers are reduced. As a result, the on-resistance of the device increases.
A second problem is a drop in destruction tolerance by a so-called latch up phenomenon. Specifically, when the IGBT 110 is turned off, a potential of the p-type well layer 160 may rise by a positive-hole current discharged through the p-type well layer 160, and the destruction tolerance of the IGBT 110 may drop by the electrons injected into the p-type well layer 160 from the n+-type source layer 148. This is because with the increase of a breaking current, the positive-hole current flowing right under the n+-type source layer 148 increases.
To solve the first problem, the use of the Injection Enhancement (IE) effect has been known in which an n-type impurity layer is formed in a lower part of the p-type well layer 160 to increase an accumulated amount of the positive hole and to reduce the on-resistance. However, when the n-type impurity layer is formed in the lower part of the p-type well layer 160, a breakdown voltage of the device itself is deteriorated. Thus, there is a limitation in raising a density of the n-type impurity layer. For the limit value, for example, an impurity total amount is in a range of about 1 to 2×1012 cm−2, and the density is in a range of about 1014 to 1015 cm−3. Therefore, there is a limitation in reduction of the on-resistance by the n-type impurity layer disposed in the lower part of the p-type well layer 160, and the on-resistance cannot be sufficiently lowered as that in a thyristor.
It is known that another means for solving the first problem comprises: further disposing dummy trenches TRd, which has no relation with the operation of the device, between active trenches TRa which contribute to the operation of the device, for example, as in an IGBT 120 shown in FIG. 28; and forming an oxide film 162 in the surface of the semiconductor layer between the dummy trenches TRd. Since this constitution interrupts contact of the semiconductor layer between the dummy trenches TRd with an emitter electrode 58, the resistance increases in discharging the positive hole to the emitter electrode 58 from an n−-type base layer 42. As a result, the accumulated amount of the positive hole can be increased. It is to be noted that to control the injection of the positive hole into the n−-type base layer 42 from the p+-type emitter layer 44 in the IGBT 120 shown in FIG. 28, an n-type buffer layer 66 is inserted between these layers. To reduce an input capacity, dummy electrodes 154 in the dummy trenches TRd having no contact with a current passage are connected to the emitter electrode 58.
However, according to researches of the present inventors, in the constitution shown in FIG. 28, a surface recombination occurs in an interface portion shown by a dot line DL between the n−-type base layer 42 and a dummy gate oxide film 172. Since the accumulated amount of the positive hole drops accordingly, it has been found out to be difficult to reduce the on-resistance.
As described above, the related-art IGBT comprising the trench gate structure has both the first problem that it is difficult to reduce the on-resistance and the second problem that with the increase of the breaking current, the positive-hole current flowing through the p-type well increases and the device may be destroyed.